It should hopefully come as no surprise that one of the most important things for plants to be able to sense and respond to is light, as that is pretty much the basis for their survival. They need sunlight to carry out photosynthesis, and they need to carry out photosynthesis in order to produce sugars to eat and survive. So, etiolation is going to be the general term for plant responses to the absence of sunlight. These responses include growing towards the sun. If you detect an absence of sunlight, your response, logically, as a plant would be to grow towards the sun. One way they might accomplish this is by having longer internodes. Remember, this is the portion of the stem between leaves. So, if you increase that length, the length of the internodes, you're going to have a lower density of leaves, but that doesn't matter because you're not getting enough sunlight anyway. These are responses to an absence of sunlight. So by having longer internodes, you're going to have more vertical growth, and that's going to allow you to reach the sun. Now, one of the last symptoms, if you want to call it that, is chlorosis, and this is a lack of chlorophyll. You can see an example of that in this very sad-looking plant right here. Now, the reason plants will experience chlorosis as a response to an absence of sunlight is actually kind of an energy conservation mechanism. Why would you want to waste energy producing chloroplasts and chlorophyll if you're not going to be catching sunlight there and thus not performing photosynthesis anyway?
De-etiolation is going to be the opposite of etiolation. It's responses to sunlight, and these are going to be in part regulated by some photoreceptors called phytochromes, and we're going to talk about phytochromes in just a little bit. Now, just because I'm piling on the terminology right here, photomorphogenesis is going to be a big focus in this lesson, and that is essentially plant growth in response to different spectra of light. Hopefully, you recall from the section on photosynthesis that there are different types of light, and plants selectively absorb specific wavelengths of that light, meaning that they're going to have responses to different types of light. Depending on the type of light they're able to receive, they will output different responses and growth patterns based on these different spectra of light, again called photomorphogenesis, and that's photo for light, morpho for form, and genesis for origin. So it's basically the origin of the form due to light. You could think of it that way.
Now, tropisms are a big category of plant responses, and these are just movements of plants in response to something in the environment. Here we're focusing on phototropism. Growth toward or away from light. So essentially, plants responding to light. However, in other lessons, we'll be looking at different types of tropisms. Phototropism is going to be controlled or is going to require, I should say, photoreceptors. These are going to be proteins that respond to stimulation from certain wavelengths of light. So if you're going to grow toward or away from light, you're going to need something to detect that light, and photoreceptors are what plants are going to use. One type of photoreceptor is, or one class of photoreceptor, I should say, are phototropins. These are blue light photoreceptors. Blue light, if you recall, is one of the main lights that plants are going to absorb for photosynthesis. In addition, they also will absorb red light. And hopefully, you also will recall that blue light is what we consider higher energy light, that is to say, it's light of a higher frequency.
Here you can see a nice example of phototropism. We have this plant here that is bending towards the light source it has here, which is a lamp here, you know, obviously could be the sun too. Now, just as it's important for plants to be able to detect, detect light they need for photosynthesis, they also need to detect when they're not getting great light for photosynthesis. So, plants use red light and blue light for photosynthesis, as I've said, and red light, the red light they preferably use is roughly in the range of 600 to 700 nanometers. And blue light, roughly around 430 to 470 nanometers. These are going to be the main bands of light that plants want for photosynthesis. It should seem logical that they are very sensitive to those particular wavelengths. They're going to respond, they're going to have strong responses to those wavelengths.
They also can detect what is called far red light. This light is not absorbed by photosynthetic pigments, meaning it's not going to help photosynthesis. It actually passes through leaves and helps indicate shade. So what that means is, high up leaves that far red light will actually pass through them, and will hit parts of the plant that are underneath those top leaves getting the direct sunlight. So it's a way for plants that are not in direct sunlight to detect that they are in shade. Right? Because the leaves above are going to absorb those photosynthetic wavelengths, but that far red light's going to make it through. So it's going to allow the plant to say, "Oh, hey, I'm not getting the good sun right now. I got to get moving here. I got to do some phototropism."
Now, plants will actually use that far red light for a really nifty thing called the red-far red switch. This is a theoretical idea that red light will promote seed germination, and far red light will inhibit it. It's based on this particular type of photoreceptor mentioned earlier, phytochrome, which is a photoreversible photoreceptor, meaning that it actually is a molecule that has two different forms, and each of those forms reacts to a different wavelength of light. When the phytochrome absorbs red light, it changes its conformation and turns into the far red phytochrome. And when this far red phytochrome absorbs far red light, that will switch it back to the red phytochrome conformation. You can see this is a nifty mechanism to detect light and shade, or dark, however you want to think of it. Now, let me get my head out of the way here, and behind my head, you can see this nifty little graph showing you the wavelengths absorbed by the red phytochrome and the far red phytochrome confirmations. The light stimulations are going to cause phosphorylations and dephosphorylations that will induce these conformational changes. Don't need to worry too much about the biochemistry, just know that when the phytochrome in the red conformation absorbs red light, it switches to the far red conformation, and when that far red conformation absorbs far red light, it flips into the red-absorbing conformation.
All of this is part of a behavior known as shade avoidance, where far red light will actually cause plants to lengthen their stems or induce branching in an attempt to grow into direct light. Lengthening the stems is great if you just need to get up to the light. Right? If the problem is your verticality. But it can also induce branching, meaning, making the plant get bushier, so it has a greater area of absorption, which is sometimes the behavior needed in order to absorb more light. And you can see that these are responses to far red light, or shade, basically. Alright, with that let's flip the page.